It has been demonstrated that air can be used, instead of water, to irrigate the aural canal during routine performance of the clinical caloric tests. Advantages obtained by the use of air are: convenience to the operator, better patient acceptance, and applicability in situations where water is contra-indicated, such as following otological surgery or in cases of tympanic membrane perforation or external otitis. However, some clinicians feel that air irrigations may sometimes fail to produce a sufficiently intense stimulation of the labyrinth, and those interested in quantitative assessment of vestibular function may question how accurately and uniformly the labyrinthine stimulus can be reproduced when using air as the irrigation fluid.
When substituting air for water as the irrigation fluid, it should be expected that certain adjustments in the technique would be required because of the marked differences in their thermal properties. For example, unless the difference between irrigation temperature and body temperature is increased, responses to air irrigations are considerably weaker than responses to water. It has been found that irrigations with air for 60 seconds at a flow rate of 8 liters per minute and temperatures of 50.degree. C and 24.degree. C were required to produce nystagmic responses equal in intensity to those obtained in the same subjects by water irrigations at 44.degree. C and 30.degree. C for 30 seconds. Furthermore, in order to ensure adequate control of air irrigation temperature it is necessary to take certain precautions in the design and operation of the irrigation equipment. Thin-walled irrigation nozzles or low flow rates may allow significant instability in the control of the temperature of the air delivered into the aural canal.
Another consideration worth noting is the change in the heat flow at the aural canal surface caused by moisture. When a wet canal is irrigated with dry air, heat is taken up by the evaporation of surface moisture. As a result, the effect of a cold air irrigation is exaggerated, and the effect of a hot air irrigation is diminished. However, repetition of the hot air irrigations will dry the canal, and thereafter the irrigations are adequate to elicit the expected response.
The necessity of irrigating a moist ear canal may arise following removal of cerumen by douching or in the presence of external otitis. Under thes circumstances, the initial irrigations with hot air may serve only to dry the canal; a second hot irrigation would then be required to produce the desired stimulation. A tympanic perforation may also interfere with the air caloric test, especially if the perforation is large. In such cases, it is important to avoid directing the air jet through the perforation because this will reduce chances of producing a proper stimulation and usually will be painful to the patient.
Finally, certain precautions seem to be indicated in view of special heat transfer properties related to "jet effects" of air. When Fitzgerald and Hallpike* introduced their method of water irrigations, they emphasized the importance of a "mass" irrigation which bathed the entire aural canal surface with water of uniform temperature, so that a controlled and repeatable heat transfer effect would be achieved. Under such conditions heat would be expected to flow uniformly in all directions away from the aural canal surface. The temperature difference developing across the lateral semicircular canal is attended with endolymph density differences which act to displace the endolymph-cupula system, thereby stimulating the receptor organ. However, when small volumes of water are used ("minimal" caloric test), care must be taken to direct the flow against the posterior-superior part of the aural canal, adjacent to the eardrum, because of the more localized conduction of heat. Otherwise, as pointed out by Dohlman*, temperature changes in the lateral semicircular canal area might be inadequate to induce nystagmus despite the presence of a healthy lateral semicircular canal organ. The result would be a "false negative" caloric reaction and the test might be misinterpreted to indicate vestibular paralysis in the ear stimulated. FNT * Fitzgerald et al, "Studies in Human Vestibular Function. I. Observations on the Directional Preponderance ("Nystagmusbereitschaft") of Caloric Nystagmus Resulting from Cerebral Lesions", Brain, 65:115-137, 1942. FNT * G. Dohlman, "Physikalische und Physiologische Studien zur Theorie des Kalorischen Nystagmus." Acta Otolaryngol., Suppl. 5,pp.1-196, 1925.
During air caloric testing the concentration of heat flow upon a small area of the aural canal could occur, in the area of jet impingement. For this reason, the operator should guard against a "false negative" test by carefully directing the jet of air against the postero-superior aural canal, preferably by visual guidance.
In order to explore the practical importance of this question, nystagmus responses to properly directed air irrigations were compared with responses to deliberately misdirected irrigations in the same ears of several normal subjects. Variations in response intensity were clearly demonstrated, though such variations were not always consistent and were not usually profound. Nevertheless, in the interest of providing uniform and repeatable caloric stimulations, care should be taken to aim the air jet at the posterosuperior aural canal surface throughout the irrigation period.
A preliminary search of the known prior art reveals the following U.S. patents as representing the best prior art found relating to the subject matter of the present invention:
Zeng, No. 1,062,698 PA1 Carroll, No. 1,106,699 PA1 Harris, No. 1,346,200 PA1 De Zeng, No. 1,588,791 McGerry No. 2,039,546
The following publications also appear to be significant in showing the known state of the art:
Albernas et al, "The Use of Air in Vestibular Caloric Stimulation", Laryngoscope, 82:2198-2203, 1972.
Capps et al, "Evaluation of the Air Caloric Test as a Routine Examination Procedure", Laryngoscope, 83:1013-1021, 1973.
Meurman et al, "The Caloric and Turning Tests After the Fenestration Operation", Acta Otolaryngol., Supp. 100, pp. 144-153,1952.
Aanta, "Caloric Test with Air", Acta Otolaryngol., Supp. 224, pp. 82-85, 1967.
Aschen, "The Caloric Test, a Nystagmographic Study", Acta Soc. Med. Upsal., 60:99-112, 1955.
Fitzgerald et al, "Studies in Human Vestibular Function. I. Observations on the Directional Preponderance ("Nystagmusbereitschaft") of Caloric Nystagmus Resulting from Cerebral Lesions, Brain, 65:115-137, 1942.
Dohlman, "Physikalische und Physiologische Studien zur Theorie des Kalorischen Nystagmus", Acta Otolaryngol., Supp. 5, pp. 1-196, 1925.